Liquid laser solution formed with a neodymium salt in phosphorus oxychloride

ABSTRACT

An improved liquid lasing solution is prepared by introducing neodymium trifluoroacetate into phosphorus oxychloride. Some Lewis acid is also desirably included in the solution in an amount to increase solubility of the neodymium III phosphorus dichloridate formed, optimize the intensity of fluorescense of the liquid lasing solution and enhance its operative efficiency. Preferably, Lewis acid of the form zirconium tetrachloride is employed and the neodymium (III) phosphorus dichloridate formed as a result of the dissolution of the neodymium trifluoroacetate in the phosphorus oxychloride is in a mole proportionality relative to the zirconium tetrachloride of not less than 1. The new liquid laser solution is highly stable over relatively long periods of time, does not degrade under flash excitation, and obviates certain undesirable absorption characteristics of comparable prior art liquid lasing compositions.

United States Patent [191 Schmitschek et al.

[ Dec. 18, 1973 LIQUID LASER SOLUTION FORMED WITH A NEODYMIUM SALT INPHOSPI-IORUS v OXYCI-ILORIDE [75] Inventors: Erhard J. Schmitschek,Munich,

Germany; John A. Trias, La Mesa, Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

221 Filed: Jan. 29, 1971 21 Appl.No.:1ll,128

[52] US. Cl. 252/3014 P [51] Int. Cl. C09k 1/36, C09k 1/62 [58] Field ofSearch 252/301.4 P, 301.3 R

[56] References Cited UNITED STATES PATENTS l/l97l Brecher et al252/3014 P OTHER PUBLICATIONS Primary ExaminerOscar R. Vertiz AssistantEtaminer-J. Cooper Attorney-R. S. Sciascia, G. J. Rubens and J. W.

McLaren 57 ABSTRACT An improved liquid lasing solution is prepared byintroducing neodymium trifluoroacetate into phosphorus oxychloride. SomeLewis acid is also desirably included in the solution in an amount toincrease solubility of the neodymium Ill phosphorus dichloridate formed,optimize the intensity of fluorescense of the liquid lasing solution andenhance its operative efficiency. Preferably, Lewis acid of the formzirconium tetrachloride is employed and the neodymium (Ill) phosphorusdichloridate formed as a result of the dissolution of the neodymiumtrifluoroacetate in the phosphorus oxychloride is in a moleproportionality relative to the zirconium tetrachloride of not lessthan 1. The new liquid laser solution is highly stable over relativelylong periods of time, does not degrade under flash excitation, andobviates certain undesirable absorption characteristics of comparableprior art liquid lasing compositions.

3 Claims, N0 Drawings LIQUID LASER SOLUTION FORMED WITH A NEODYMIUM SALTIN PI-IOSPI-IORUS OXYCIILORIDE STATEMENT OF GOVERNMENT INTEREST Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The first liquid laser compositionsdeveloped had the inherent disadvantage of requiring operation atextremely low temperatures for most efficient lasing results. Followingthe initially developed liquid lasing compositions, improved liquidlasing solutions were devised which afforded efficient laser operationat substantially room temperatures, i.e., in the temperature range of-40C to +30C, as exemplified and disclosed by U. S. Pat. No. 3,360,478,issued Dec. 26, I967 to Erhard J. Schimitschek et a1.

Still more recently, improved liquid laser solutions were developedincluding neodymium salt dissolved in phosphorus oxychloride asdisclosed by Erhard J. Schi mitschek in the Journal of Applied Physics,Volume 39, No. 14, of December 1968. That publication disclosed thetechniques employedand the results obtained from a liquid lasingsolution developed by dissolving neodymium perchlorate and titantiumtetrachloride in phosphorus oxychloride. The neodymium perchloratedissolved in the phosphorus oxychloride produced characteristic Ndfluorescence, but the intensity was not as strong as might be expectedfrom a solvent which has its highest vibrational frequency at 1300cm.However, the fluorescen se intensity was increased markedly, byapproximately the order of fifty times, when small amounts of a Lewisacid, such as titanium tetrachloride, were added.

The 1968 Schimitschek disclosure in the Journal of Applied Physics,referred to herein, disclosed in part that a typical liquid lasersolution may be prepared by dissolving a calculated amount of neodymiumperchlorate in a :1 mixture (by volume) of phosphorus oxychloride andtitanium tetrachloride to givea 0.1M'solution. It was, however,necessary that all manipulations be carried out under a fume hoodbecause of the highly corrosive nature of the chemicals employed.

Additionally, it was found that after several weeks of operation theseliquid lasing compositions had the disadvantages of deteriorating,becoming cloudy, and also exhibiting large amounts of backgroundabsorption in the blue pump region.

SUMMARY OF THE INVENTION The present invention comprises an improvedliquid lasing composition consisting essentially of neodymium (III)phosphorus dichloridate, retained in solution with phosphorusoxychloride by the addition of a Lewis acid. Preferably the solutionincludes Lewis acid in sufficient amount to increase solubility, andoptimize the intensity of fluorescense of the composition therebyenhancing its operative efficiency. The most desirable form of Lewisacid has been found to be zirconium tetrachloride and in a preferredform of the liquid lasing composition of the present invention,zirconium tetrachloride relative to the neodymium (III) phosphorusdichloridate formed by dissolution of the neodymium trifluoroacetate inthe phosphorus oxychloride is in a mole proportionality, of not lessthan one.

In other words, the most desired proportionality in the new liquidlasing composition may be expressed as follows:

[Nd(PO Cl- [ZrCh], complex dissolved in POCl (n/m 2 l In a preferredform of the new liquid laser composition it has been found that the Ndconcentrations are typically 1 to 2 X 10 ions/cm and if the compositionis kept moisture free, the solution has been found not to degrade withthe passage of time nor under flash excitation conditions.

Thus, the liquid laser composition of the present invention is asignificant improvement over prior, known comparable compositions. It issubstantially free of deterioration due to aging or due to flashexcitation. By contrast comparable prior art liquid laser solutionsbecame cloudy with time and also exhibited undesirable absorptioncharacteristics.

Accordingly, it is an object of the present invention to provide anefficient liquid lasing composition which is more stable than comparableprior art liquid lasing compositions.

Another most important object of the present invention is to provide anew liquid lasing solution which does .not deteriorate with time nor asa result of flash excitation.

Yet another most important object of the present invention is to providea liquid lasing solution which minimizes undesirable absorptioncharacteristics exhibited by comparable prior art liquid lasingsolutions.

These and other features, advantages, and objects of the presentinvention will be better appreciated from an understanding of thedisclosure of a preferred form of the new liquid lasing solution whichfollows.

DESCRIPTION OF THE PREFERRED EMBODIMENT The new liquid lasing solutionis essentially a composition of neodymium (III) phosphorus dichloridateand zirconium tetrachloride dissolved in phosphorus oxychloride. The Nd(III) phosphorus dichloridate is most conveniently formed by introducingNd trifluoroacetate into the phosphorus oxychloride with which it reactsand forms the Nd(III) phosphorus dichloridate. Solutions prepared inaccordance with the teachings of the present invention proved to bestable over a test period of approximately 1 year and the undesirableabsorption characteristics of prior art comparable solutions wereessentially obviated, the pump absorption being due to Nd alone. Thesolvation process involved may be described as follows:

POCl where n/m is 2 1.

During the lasing action, the measured fluorescense decay time wastypically 325 microseconds and the quantum efficiency was 47 i 5%.During the laser experiments a teflon pump was employed to continuouslyrecirculate the lasing liquid through the laser cell and also through anexternal heat exchanger fabricated of nickel. High flow velocities werereadily achieved because the viscosity of the lasing liquid was only 1.7to 2.7 centipoise, depending on the doping level. The laser cell wasmade of fused quartz tube mm long and 7mm in diameter. With externalflat mirrors 60cm apart, the dynamic losses were determined to be0.013cm". With spherical mirrors having a radius of 1.47m in the sameconfiguration, the losses could effectively be reduced to 0.003cm". Thethreshold energy was found to be 7 joules and the slope efficiency 0.6%.However, with rigorous exclusion of moisture in the preparation of thesolution, the scattering could be further reduced and slope efficienciescomparable to Nd doped glasses are obtainable.

With flat mirrors, slightly above the threshold, clearly defined modepatterns were observed in the output. When the optical pump power wasincreased, the beam broke into many irregularly distributed hot"filaments. Time resolved studies indicate that the hot spots" changetheir location within the beam in less than one microsecond. Themeasured beam divergence of the output beam with flat mirrors was 1.2 X10 radians. This is about seven times defraction limited if related tothe 7mm large aperture of the laser cell. It becomes defraction limitedif related to the average size of the filaments which were between 0.5and 1mm.

In the frequency domain, the output consisted of one single line at1.054 microns. Excitation spectra of the laser solution taken atdifferent temperatures down to that of liquid helium clearly show thatthe fluorescing Nd-complex is present in solution in at least threedifferent species. The fact that still only one laser output line can beobserved suggests a fast enough crossrelaxation among the species togive the appearance of a homogeneously broadened line.

Stimulated Raman emission emerging simultaneously with the main laseremission could be observed only when the laser was passivelymode-locked. The Stokeshifted Raman line occurred at 488cm in very goodagreement with one of the Raman-active stretching vibrations of POCl at483cm". Extensive experimentation demonstrated that repetitionfrequencies of Spps could readily be achieved with several megawattspeak output power (actively Q-switched) and construction of a largersystem could reasonably be expected to significantly increase the poweroutput.

Thus, the liquid lasing composition of the present invention obviatesseveral disadvantages of comparable prior art liquid lasing solutions.Most significantly, the new liquid lasing composition has proven to behighly stable and not subject to becoming cloudy, either due to thepassage of time or due to flash excitation during the lasing process.

Moreover, the new liquid lasing solution in addition to being capable ofproducing highly desirable laser emission, has proven to havesubstantially eliminated the highly undesirable large backgroundabsorption in the blue pump region that was characteristic of neodymiumperchlorate and titantium tetrachloride dissolved in phosphorusoxychloride, one of the typical comparable prior art liquid lasersolutions.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A liquid lasing solution consisting essentially of neodymium (lll)phosphorus dichloridate and zirconium tetrachloride dissolved inphosphorus oxychloride, wherein said zirconium tetrachloride is in anamount to optimize the solubility and fluorescence of the neodymium(lll) phosphorus dichloridate.

2. A liquid lasing solution as claimed in claim 1 wherein said zirconiumtetrachloride is in a mole proportionality relative to said neodymium(lll) phosphorus dichloridate of not less than one.

3. A liquid lasing solution as claimed in claim 1 wherein the neodymium(D-phosphorus dichloridate is formed by dissolution of neodymiumtrifluoroacetate in phosphorus oxychloride.

P en N 3,779,939 Da d December 18, 1975 Inventor(s) ERHARD J.SCHIMITSCHEK ET. AL.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

On the cover sheet [19] "Schmitschek" should read Schimitschek Also onthe cover sheet [75 "Schmitschek" should read Schimitschek Signedandsealed this 13th day of August 1974.

(SEAL) Attest:

McCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM (10459) A USCOMM-DC 60376vP69 n U. 5. GOVERNMENT PRINTINGOFFICE 1 99 0*!356'33.

2. A liquid lasing solution as claimed in claim 1 wherein said zirconiumtetrachloride is in a mole proportionality relative to said neodymium(III) phosphorus dichloridate of not less than one.
 3. A liquid lasingsolution as claimed in claim 1 wherein the neodymium (III)-phosphorusdichloridate is formed by dissolution of neodymium trifluoroacetate inphosphorus oxychloride.